The invention relates to the field of optical gap measuring.
X-ray lithography is used for printing circuit features down to sub-100 nm dimensions. Proximity printing is generally used, in which an x-ray mask is separated from the substrate to be printed by a gap of generally less than 40 xcexcm, and often less than 10 xcexcm. The technology is developing toward requiring gap control to a tolerance within 100 nm and overlay alignment between successive printings of better than 10 nm. These tolerances are expected to become yet smaller as the technology develops.
There are great advantages in a scheme that requires no hardware in the path of the x-ray beams that are used to print the pattern from the mask to the substrate, and uses the same hardware as for the aligning function. Also, a scheme that does not require aligning before gapping is desired.
A scheme, referred to as Interferometric Broad-Band Imaging (IBBI) which meets the requirements for aligning has previously been developed and demonstrated within the MIT Nanostructures Laboratory for aligning overlay between successive printings to sub 10-nm accuracy, with limited capability for gap-measuring. This scheme is described in U.S. Patent Nos. 5,414,514 and 5,808,742.
Optical gap-measuring has been more elusive than aligning. Complications arise from the abundance of different light-paths, including those within the mask thickness, coupled with the high sensitivity to small effects when using coherent light.
One class of potential solutions has required mechanical scanning of the gap while observing a fluctuating intensity from optical interference. One approach in this class is described in U.S. Pat. No. 5,808,742. This class has the disadvantage that too much time is taken by the mechanical scanning, and the scanning can not take place during the x-ray exposure. However, such a method can be very useful for occasional calibration check, and is built into the present invention.
Another class relies on imaging of fringes. This is more attractive since it overcomes the problems introduced by the scanning. Also, it complements the IBBI aligning approach which is the subject of the above mentioned patents.
The invention represents an improvement over the gapping, i.e., gap-measuring, capabilities described in U.S. Pat. No. 5,808,742,and its subsequent divisional application Ser. No. 09/150,426, both of which are incorporated herein by reference.
An objective of the invention is to provide a system and process of measuring the gap between one substantially planar object, such as a mask, and a second planar object, such as a substrate, with a higher degree of sensitivity, accuracy, capture range, and reliability, than previously obtained. Other objectives are to achieve these goals while: a) requiring no mark on the substrate (for initial lithographic printing); b) not blocking the volume through which a lithographic exposing beam must pass; c) not requiring aligning before gapping; d) sharing optics also used for aligning of lithographic overlays; f) sharing the same field of view used in the aligning function of these optics; g) sharing imaging hardware and processing techniques used in the aligning; and h) providing rapid measuring capability consistent with production requirements. The invention provides a scheme that meets all the objectives and overcomes all the problems discussed above.
In particular, the invention uses the same hardware and inclined illuminating path as the complementary IBBI overlay-aligning scheme referred to above. In addition, a mark containing the gapping features may be combined with a mark containing the aligning features, so that both gapping and aligning features in the mark may be viewed within a single field-of-view of the imaging sensor in the IBBI/gapping optics. Spatial filtering, beneficial to both the aligning and gapping, can also be shared. The image-processing for IBBI and gapping will also be similar. It will be clear that this combination of functions will be valuable in a lithographic tool that handles both aligning and gapping.
According to the invention, gapping marks on a first plate, such as a mask, each contain one or more 2-dimensional gratings. Periodic structures, such as gratings, are commonly described by means of vectors that point along directions in which the structure is periodic. Thus, a two-dimensional grating can be described by two or more vectors. In this invention, each 2-D grating typically has a constant-period vector in the plane of incidence, i.e., the plane that contains the input beam and the normal to the first plate, and a varying-period grating-vector in the transverse plane. These gratings are fabricated on the surface of the mask facing the second plate, such as a substrate. Typically the gapping portion of a mark will have gratings only on the mask, and not on the associated part of the substrate-mark, to ensure that the relative misalignment is isolated from the gapping-measurement. In some cases the gratings could be on the substrate alone, but this would generally be less advantageous.
The varying period, sometimes called chirp, in the transverse plane is an innovative and unique aspect of the invention. Each of these 2-D gratings is typically formed in the shape of a narrow stripe, with the shorter dimension in the plane of incidence. For some applications, this shape may be varied.
The constant period in the plane of incidence permits the use of illuminating light at an inclined angle, and so removes the need for optical components in the path of the lithographic exposing beam. An inclined light source (as used by the IBBI), is arranged to illuminate these gratings, and the surface of the second plate, to produce two images of each grating, viewed on the same or similar inclined path as the illumination. The direct image will be referred to as the primary image. The secondary image corresponds to an image of the grating that has been reflected in the surface of the substrate. The distance apart of these images will yield the gap on a relatively coarse scale, to an accuracy within about one micron. The latter will provide a quick and useful measurement for ensuring avoidance of collision as the mask initially approaches the substrate.
Because of the varying transverse grating-period, the images also contain fringes, resulting from interference between paths having traveled different distances through the gap and the mask-plate as a result of successive diffractions and reflections. Without the grating-period variation, these interferences would cause each of the two images to have image intensity that is uniform at any one gap, but the intensity would fluctuate with changing gap. The varying transverse grating-period introduces varying transverse path-differences, which in turn result in interference-fringes in the images. It is found that the phases and periods, and particularly the phases, of these fringes vary with gap. Moreover, at larger gaps, when the primary and secondary images are distinguishable, it is found that the primary image has its phase changing with gap at a much higher rate than that of the secondary image. Measurements of the fringe-phases, coupled with measurement of the geometric separation of the two images, will yield an accurate measure of the gap over a wide range.
For direct self-contained calibration, a linear grating, with uniform period, is also included within the gapping mark. It uses an Inclined Diffractive Michelson (IDM) technique. This technique uses the same inclined illuminating path and hardware as used in the present invention and the complementary IBBI overlay-aligning scheme referred to above. It requires scanning of the gap, but this is acceptable for the initial calibration, and for subsequent occasional verifications. The inclusion of this self-contained calibration capability with the gapping function is another innovative aspect of the invention.
Additional gratings may be added, within the gapping mark, to extend the range of gap-measurement without ambiguity, and also to improve the accuracy of measurement. These additional gratings, like the one initially described, will have uniform grating-period in the plane of incidence, and different varying grating-periods in the transverse plane. These additional gratings will be of two varieties. The first, for improving accuracy, will be identical with an existing one except that the direction (or sense) of the varying period will be reversed. This will cause the phase of the resulting fringes to move in the opposite direction (or sense). Measuring the phase-differences between the two sets of contra-moving fringes will result in a more accurate measure of the gap. The second variety, for removal of ambiguity, will have a different rate of transverse grating-period change. This will result in a different gap-cycle, leading to a different set of ambiguities. This allows the correct measurement to be selected from different series of ambiguities by cross-matching the gap values.
The extra gratings assist in expanding the range of accurate gap-measuring down to very small gaps (i.e. to  less than 20 xcexcm which are important for x-ray lithography) where the primary and secondary images coalesce and their fringes can no longer be separately identified. The slower-moving fringes of the secondary image are thus absent and no longer available for breaking the ambiguities in the measurement obtained from the more rapidly moving fringes that still remain.
This gapping method requires a corresponding area on the adjacent substrate surface to be free of any feature that will have significant diffracting properties that might cause interference with the images that are used for the gapping. The presence of photoresist or other coating will affect the measurement. However, this will not be a problem provided the calibration is done with a similar coating. Alternatively, a table of offsets may be maintained to correct for such layers.